Sounds kinda weird.. but studying nearly identical coral reef systems off Australia, my collaborators and I discovered something unusual on the reefs subjected to nearly exclusive fishing of sharks—fish with significantly smaller eyes and tails. This provides evidence of body shape changes in fish due to human-driven shark declines from overfishing.
In the study, our research team analyzed seven different fish species from two neighboring coral reef systems off the coast of northwestern Australia. The coral reef systems, known as the Rowley Shoals and the Scott Reefs, are each comprised of multiple atoll-like reefs, are nearly identical biologically and physically in all but one way— the coral reefs in Rowley Shoals are protected from fishing, while the coral reefs in the Scott Reefs have been subjected to nearly exclusive commercial shark fishing for centuries. Targeted shark fishing has intensified in the region in recent decades to fuel the demand of shark fin soup. As a result, shark populations have been decimated at the Scott Reefs, but remain healthy at the Rowley Shoals.
Our team collected 611 fish of seven different species across multiple sites from different coral reefs within the Rowley Shoals and the Scott Reefs. We then took photographs of each fish and digitally analyzed photographs, measuring body length, body width, eye area and tail area of each fish.
We found that at Scott Reefs, where shark populations have declined, the eyes of fishes that are normally prey for sharks were on average up to 46 percent smaller compared to the same sized fish of the same species on reefs at the Rowley Shoals where shark populations are healthy. The same pattern was true for fish tail sizes, with the overall size of fish tails being on average up to 40 percent smaller at the Scott Reefs compared to the Rowley Shoals. Interestingly, these patterns were consistent across seven fish species that vary in behavior, diet and trophic-guild.
Eye size is critical for detecting predators, especially under low-light conditions when many sharks usually hunt, and tail shape enables burst speed and rapid escape from sharks. So we believe that the removals of sharks by humans have potentially caused a reduction in the size of fish body parts that are otherwise normally important for detecting and avoiding sharks. It’s possible that the removal of sharks have lead to an evolutionary change in the fish.
The differences in fish body shapes measured between the two coral reef systems could also have consequences for energy flow throughout the ecosystem, ultimately impacting the food web. These results are particularly important since sharks are among the most threatened marine animals and the consequences of their global removals due to fishing is not well understood and has been a topic of significant speculation, debate and concern. However, these finding shed new light on our understanding of the potential cascading effects the loss of sharks on marine ecosystems.
Research paper: Hammerschlag N, Barley SC, Irschick DJ, Meeuwig JJ, Nelson ER, Meekan MG (2018) Predator declines and morphological changes in prey: evidence from coral reefs depleted of sharks. Marine Ecology Progress Series 586:127-139.